In thermal power plants, for example, carbon dioxide (hereinafter, referred to as CO2) is produced with combustion of fossil fuels such as coal to increase the CO2 concentration in the atmosphere. It has been said that this is accompanied by an increase in temperature which causes various environmental issues. Since Kyoto Protocol has been adopted at Kyoto Conference of Parties to the Framework Convention on Climate Change (COP3) in December, 1997 in order to prevent global warming, countermeasures for reducing the amount of CO2 emission have been conducted in each country. Among methods recovering CO2 from combustion exhaust gas containing oxygen (O2) and sulfur oxides (SOX) discharged from thermal power plants, etc., absorption by an alkanolamine solution is most likely to be put into practical use at present (Patent Literature 1). Regarding this method, types of alkanolamines suitable for CO2 recovery (e.g., Patent Literature 2) and conditions for applying the method to exhaust gas containing sulfur oxides (e.g., Patent Literature 1) have been studied. These alkanolamines are harder to be oxidized with oxygen contained in combustion exhaust gas than conventionally used monoethanolamine, however, in light of long period of use longer than several thousand hours, a problem of replacement amounts of these solutions due to oxidative degradation occurs. Against this problem, oxidation of an alkanolamine in the absorbing solution can be considerably inhibited by adding to the solution an organosulfur compound, such as a mercaptoimidazole or a mercaptobenzimidazole, as an oxidation inhibitor for the alkanolamine (e.g., Patent Literature 3).
In a method of recovering CO2 contained in combustion exhaust gas from a boiler, CO2 is recovered by bringing the gas to be processed containing CO2 into contact with an absorbing solution of an alkanolamine inside an absorption column to give a CO2-rich solution and subsequently circulating the solution in a regeneration column to thermally release CO2 (e.g., Patent Literature 1).